Heat 3⁄4 cup cream in a 1 litre heavy saucepan until hot. Whisk together the Irvine’s Egg yolks, sugar, and a pinch of salt in a bowl until combined well, then add the hot cream in a slow stream, whisking until combined. Transfer the mixture to the saucepan and cook over moderately low heat, stirring constantly, until it registers 70°C on a thermometer. Pour the custard through a fine- mesh sieve into a bowl and stir in vanilla essence.
Melt chocolate in a metal bowl set over a pan of simmering water, stirring frequently. Whisk the custard into the chocolate until smooth, then cool.
Beat the remaining 11⁄4 cups cream in a bowl with an electric mixer until it just holds stiff peaks. Whisk one fourth of the cream into the chocolate custard to lighten, then fold in remaining cream gently but thoroughly.
Spoon mousse into 8 glasses or ramekins and chill, covered, for at least 6 hours. Let stand at room temperature about 20 minutes before serving.